Diagnosis of the Kinetic Energy of the “21·7” Extreme Torrential Rainfall Event in Henan Province, China

An extreme torrential rain (ETR) event occurred in Henan Province, China, during 18-21 July 2021. Based on hourly rain-gauge observations and ERA5 reanalysis data, the ETR was studied from the perspective of kinetic energy (K), which can be divided into rotational wind ( V _R) kinetic energy (K_R), divergent wind kinetic energy (K_D), and the kinetic energy of the interaction between the divergent and rotational winds (K_RD). According to the hourly precipitation intensity variability, the ETR process was divided into an initial stage, a rapid increase stage, and maintenance stage. Results showed that the intensification and maintenance of ETR were closely related to the upper-level K, and most closely related to the upper-level K_R, with a correlation coefficient of up to 0.9. In particular, the peak value of hourly rainfall intensity lagged behind the K_R by 8 h. Furthermore, diagnosis showed that K transformation from unresolvable to resolvable scales made the ETR increase slowly. The meridional rotational wind (u_R) and meridional gradient of the geopotential (φ) jointly determined the conversion of available potential energy (APE) to K_R through the barotropic process, which dominated the rapid enhancement of K_R and then caused the rapid increase in ETR. The transportation of K by rotational wind consumed K_R, and basically offset the K_R produced by the barotropic process, which basically kept K_R stable at a high value, thus maintaining the ETR.